Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a recording medium using an electrophotographic technique.
Description of the Related Art
Conventionally, in image forming apparatuses such as laser beam printers, an in-line color system is known in which a plurality of image forming stations are arranged in parallel in a movement direction of an intermediate transfer belt. With an image forming apparatus adopting the in-line color system, first, an electrostatic latent image is formed on a surface of a photosensitive drum in the plurality of image forming stations. The electrostatic latent image formed on the photosensitive drum is developed by a developing apparatus as a toner image. In addition, toner images of respective colors formed in the plurality of image forming stations are primarily transferred onto the intermediate transfer belt so as to overlap with each other. The toner images of the respective colors primarily transferred onto the intermediate transfer belt are then secondarily transferred to a recording material such as a sheet of paper. Subsequently, as the recording material to which the toner images have been secondarily transferred is heated and pressurized by a fixing apparatus, the toner images are fixed to the recording material. In this manner, an image is formed on the recording material. In this case, density of the image to be formed on the recording material is desirably consistent with density desired by a user. In addition, a tinge of the image to be formed on the recording material is also desirably consistent with a tinge desired by the user.
In consideration thereof, in a technique disclosed in Japanese Patent Application Laid-open No. H8-227222, a tinge of an image to be formed on a recording material is adjusted by increasing an amount of toner conveyed from a developing sleeve of a developing roller to a photosensitive belt (a belt-shaped photoreceptor). Specifically, in the technique disclosed in Japanese Patent Application Laid-open No. H8-227222, the developing roller includes the developing sleeve and a magnet roller configured to be rotatable inside the developing sleeve. By increasing a rotational speed of the magnet roller, a toner amount conveyed from the developing sleeve to the photosensitive belt is increased.
Furthermore, conventionally, a technique is known for increasing a tinge selection range (a color gamut) or increasing density of an image to be formed on a recording material by varying a peripheral velocity difference between a photosensitive drum and a developing roller. In a technique disclosed in Japanese Patent Application Laid-open No. 2013-210489, a color gamut of an image is enlarged and an upper limit value of density of the image is increased by varying a peripheral velocity difference between a photosensitive drum and a developing roller. In addition, the technique disclosed in Japanese Patent Application Laid-open No. 2013-210489 suppresses toner scattering, image thinning, and the like which are caused when the peripheral velocity difference between the photosensitive drum and the developing roller is increased. Specifically, instead of increasing the peripheral velocity difference between the photosensitive drum and the developing roller by increasing a peripheral velocity of the developing roller, the peripheral velocity difference between the photosensitive drum and the developing roller is increased by reducing a peripheral velocity of the photosensitive drum. Accordingly, toner scattering, image thinning, and the like are suppressed.
However, in recent years, there are demands to approximate an image formed by an image forming apparatus to an image displayed on a display. In other words, there are demands to enlarge a color gamut of an image to be formed on a recording material. In order to do so, a peripheral velocity ratio between a photosensitive drum and a developing roller must be increased. This can be realized by providing a printing operation for enlarging a color gamut separately from a normal printing operation. In this case, in the printing operation for enlarging a color gamut, the peripheral velocity ratio between the photosensitive drum and the developing roller is set larger than in the normal printing operation.
However, when the peripheral velocity ratio between the photosensitive drum and the developing roller is increased, toners slide against each other and become vulnerable to degradation. When the printing operation for enlarging the color gamut is performed over a long period of time, the degradation of the toners may result in creating a defect in an image. When toner is consumed rapidly, since there is less degradation of the toner when the toner is used up, an image can be formed on a recording material in a preferable manner. However, when the toner is consumed slowly, since the toner degrades before the toner is used up, a defect may occur in an image formed on a recording material. In this case, the degradation of the toner can conceivably be reduced by having the user himself/herself switch between the normal printing operation and the printing operation for enlarging the color gamut. However, since such settings must be performed by the user himself/herself, usability declines.